A combined inforamtion recording and graphic display device is described herein, which is intended to serve electronically the function until now performed by instruments known as "chart recorders".
Chart recorders are used throughout industry to provide a permanent record of a varying electrical signal. Such electrical signal is converted, upon being fed to the recorder, to cause displacement of a chart marking instrument such as a pen, which is displaceable in a line across the chart whilst the chart itself is displaced, perpendicularly with respect to the line of movement of the pen, at a constant rate or incrementally in dependence on the circumstances. The combined effect of the movement of the pen or other marking instrument, which represents the value of the input signal at any instant in time, and the movementof the paper within the chart recorder, produces a varying line constituting a permanent record of the value of the input signal at any instant in the time period recorded by the instrument. Such permanent records may be required for any one of a number of purposes, for example analyzing the past history of variation of a parameter sensed by a suitable sensor and converted into an electrical signal which is supplied to the chart recorder, and may be used for any purpose for analysis, control or prediction.
A major disadvantage with chart recordoers is constituted by the sheer bulk of the paper constituting the record of the signal variations. Indeed, given that the chart marking usually constitutes only a single line on a chart of fixed width the major part of the area of the chart is wasted in the sense that it provides no useful information except to constitute a frame of reference within which the value of the signal can be determined.
If the information contained on the chart is likely to be needed again the whole chart must be stored and the storage problems for such bulky objects rapidly become a major problem. Acess to a given item of information is also difficult since the chart can only be viewed sequentially by "scrolling" through the length of the chart to find a period of interest.
Other disadvantages of known chart recorders are related to their mechanical nature, and therefore the relatively poor response to rapidly fluctuating input signals which limits the high frequency end of the signal spectrum, and to the fact that mechanical disturbances to the recording instrument itself can affect the value of the record thus produced independently of the variations in the input signal thereby introducing another error. Further, the relatively fixed nature of the chart record means that comparison between the signal values at tow widely differing points in time can be effected only with difficulty, either by laying the charts out on a large table and physically moving the regions of interest into juxtaposition, or by effecting measurements of the linear dimensions on the chart and storing these separately thereby, again, introducing potential errors in measurement of the chart itself.